This invention relates to apparatus for supporting relatively thin plates of material having opposed parallel surfaces, such as semiconductor wafers, for processing or testing in a metrology system.
In the manufacture of devices from semiconductor wafers, such as silicon wafers, the production and quality control processes require a precise knowledge of the characteristics of the wafer, such as its flatness, its thickness, and other characteristics. Particularly important are accurate profiles of the surfaces of the wafer in conjunction with measurements of the shape and thickness of the wafer at all points on its surface. Current processing requires, for many situations, profiling and flatness measurements of both the front and back surfaces of such wafers.
In the past, measurements of thickness variations were accomplished by means of capacitive probes and the like, such as disclosed in the United States patent to Abbe U.S. Pat. No. 4,860,229. As disclosed in this patent, a wafer is mounted on a rotatable vacuum chuck in a wafer flatness station; and a capacitive probe is placed in a position to provide outputs indicative of the wafer thickness as the wafer is rotated beneath the chuck. The data which is provided by the capacitive thickness sensor then is provided to a processor for computing a flatness profile of the wafer.
Optical profiling of semiconductor wafers also has been effected by means of interferometric systems using phase shifting to produce a profile of the wafer. In systems which have been used in the past, such profiling typically employed a vacuum chuck to hold the wafer by attracting its reverse side to the chuck, which ostensibly is a flat plane. However, any variations in the flatness of the plane of the vacuum chuck surface are imparted directly to the wafer, since it is highly flexible. In addition, if the wafer itself is naturally bowed, the pulling of the vacuum chuck on the wafer will remove the bowing; so that an accurate profile or flatness measurement of the wafer as it actually exists does not occur. Current wafers are being manufactured in ever increasing diameters, many ranging between 200 mm or 300 mm in diameter (approximately 8″ or 12″); so that when such a wafer is placed on a surface or is held horizontally at its edges, it, tends to sag under the effects of gravity, thereby making accurate flatness and profiling measurements difficult, if not impossible. This deformation of the wafer may be incorporated in the measurement results; so that its flatness and thickness cannot be obtained with sufficient accuracy.
Another problem with using vacuum chucks to hold the wafer during the profiling or measuring operations, whether capacitive measurements or interferometric optical measurements are being used, is that there is a physical contact between the vacuum chuck and the surface of the wafer adjacent the vacuum chuck. This can result in the impartation of defects to the wafer from the vacuum chuck itself.
Current semiconductor processing frequently requires semiconductor wafers which are polished on both surfaces. Thus, it is desirable to provide flatness measurement and profiling of both sides of the wafer. In the past, this frequently has been accomplished with an interferometer by holding the wafer, such as in a vacuum chuck as mentioned above, in one position, to allow the optical scanning of one side of the wafer. After the wafer has been scanned on one side, it then is physically reversed and placed back in the interferometer for scanning the opposite side. Obviously, this sequential processing is time consuming. The movement and physical repositioning of the wafer which is necessary also makes it very difficult to obtain accurate thickness variation measurements of the wafer, since the manipulation subjects the entire process to potential error. The flatness measurement and profiling of opposite sides of a wafer in a sequential manner also more than doubles the processing time which is required when only one surface is to be subjected to the flatness measurement and profiling.
Two United States patents, to Abe U.S. Pat. Nos. 5,995,226 and 6,504,615, purport to show an optical apparatus to simultaneously measure both surfaces of a semiconductor wafer. In the disclosures of both of these patents, a wafer is shown as positioned vertically between a pair of identical interferometers, which then provide signals to a computer or a pair of computers representative of the flatness and profile of the opposing front and back surfaces of the wafer. In neither of these patents is there any disclosure of the manner in which the wafer is held vertically in order to allow the simultaneous optical or interferometric measurement of the two sides of the wafer.
Two World Intellectual Property Organization patents, to Mueller et al., No. WO 01/77612 A1, and to Sullivan et al., No. WO 00/79245 A1, purport to show an optical apparatus for a similar purpose, where a semiconductor wafer is positioned vertically while both surfaces are simultaneously presented for optical analysis. In both patents, there is disclosure of a method of support of the wafer using an on-edge three-point kinematic mount consisting of clips having spherical or semi-spherical tangentially mounted contacts, mounted to a support plate and arranged to be substantially coplanar, where the clips are adjustable to provide for slight irregularities in the shape of the wafer. There is no disclosure made as to the method and apparatus of clip adjustment, nor is there disclosure made as to the method and apparatus for the loading and unloading of the wafer to and from the clips, nor is there disclosure made as to the method and apparatus for compensation of normal production variations in wafer thickness and diameter.
An important requirement for the shape metrology of wafers is the measurement of the intrinsic shape, i.e. the shape without any external forces acting on the wafer. The shape of thin, large diameter wafers is very easily distorted by external forces, by gravitational forces, as well as by forces introduced by the holding mechanism. Gravitational effects are best minimized by holding the wafer in a vertical position where the gravitational force vector is in the wafer plane. However, standard wafer handling equipment handles wafers in a horizontal orientation. Additionally, in order to avoid or minimize holding effects on the wafer shape, special care has to be taken in the design of the holding mechanism.
In highly sensitive metrology systems, vibrations of the wafer or test piece are detrimental to the measurement process. The main vibration mode of wafers consists of bending vibrations with excursions normal to the wafer plane, i.e. the wafer shape fluctuates during vibrations. Thus, a mount optimized for not affecting the wafer shape cannot easily affect the vibrations of the wafer. Ambient vibration is ever-present in the metrology process in that the sources of acoustic and seismic periodic displacement are many; they may emanate on a continuous basis and in an unpredictable manner from facility foundations and floors, walls, climate control systems, nearby process equipment and machinery, and from the very equipment and mechanisms used to support and perform a particular metrology process. While a variety of vibration attenuation methods are commonly employed to reduce the effects of vibration on the metrology process, such as actively damped equipment pedestals and supports and passive dampers of numerous varieties, not all energy is dissipated before it is transmitted to the wafer. Additionally, air motion in the vicinity of the wafer can impart vibration directly to the wafer in that a large, thin, semi-rigid sheet of material can become a resonating membrane when it is supported on its edge.
It is desirable to provide an apparatus for holding wafers or other thin objects that stably and accurately holds the object in a vertically oriented position while minimizing the application of distorting stress to the wafer, and attenuates vibration transmitted from the environment to the holding apparatus while additionally attenuating vibration of the object at the point of contact for subsequent processing, such as interferometric profiling.